I think you are confusing the rules of the Perfect tense (using avoir or être) with this use of the verb être simply followed by an adjective.
The use of the imperfect is correct as être, being a verb of state, will be more often in the imperfect ( imparfait) than the Perfect (passé composé) but if it were it would be
Il a été bouleversé par cette nouvelle ( indicating that he has probably got over it by now)
Hope this helps!
If you are talking of a person you could say -
Il était anéanti/bouleversé/effondré ...
if you are talking of a building, land, country ...
you could use dévasté/ ravagé.
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